This invention relates to the field of borehole measurement. More particularly, this invention relates to the field of measurement while drilling (MWD) and to a method of measuring the parameter of azimuth while the drill string is rotating.
In MWD systems, the conventional approach is to take certain borehole parameter readings or surveys only when the drillstring is not rotating. U.S. Pat. No. 4,013,945, owned by the assignee hereof, discloses and claims apparatus for detecting the absence of rotation and initiating the operation of parameter sensors for determining azimuth and inclination when the absence of rotation is sensed. While there have been several reasons for taking various MWD measurements only in the absence of drill string rotation, a principal reason for doing so is that previous methods for the measurement or determination of angles of azimuth and inclination required the tool to be stationary in order for the null pints of single axis devices to be achieved; or to obtain the averaging necessary when triaxial mangetometers and triaxial accelerometers are used for determining azimuth and inclination. That is, when triaxial magnetometer sand accelerometers are used, the individual field measurements necessary for determination of azimuth and inclination are dependent on instantaneous tool face angle when the measurements are taken. This is so because during rotation the x and y axis magnetometer and accelerometer readings are continually varying, and only the z axis reading is constant. In referring to x, y and z axis, the frame of reference is the borehole (and the measuring tool), with the z axis being along the axis of the borehole (and tool), and with the x and y axes being mutually perpendicular to the z axis and each other. That frame of reference is to be distinguished from the earth frame of reference of east (E), north (N) (or horizontal) and vertical (D) (or down).
There are, however, circumstances where it is particularly desirable to be able to measure azimuth and inclination while the drillstring is rotating. Examples of such circumstances include (a) wells where drilling is particularly difficult and any interruption in rotation will increase drill string sticking problems, and (b) situations where knowledge of instantaneous bit walk information is desired in order to know and predict the real time path of the borehole. A system has heretofore been proposed and used for obtaining inclination while the drillstring is rotating. In addition, U.S. patent application Ser. Nos. 054,616, now issued as U.S Pat. No. 4,813,274 and 054, 552, now issued as U.S. Pat. No. 4,894,923 both filed on May 27 1987, disclose methods for obtaining azimuth measurements while rotating. Both applications are assigned to the assignee hereof, and fully incorporated herein by reference.
Unfortunately, measurement of rotating azimuth and inclination disclosed in U.S. application Ser. Nos. 054,616 and 054,552 suffer from a number of problems. The inclination (as disclosed in application Ser. No. 054,616) suffers from sensitivity problems at low inclination as well as acquisition problems due to occasional accelerometer channel saturation while drilling. Inclination while rotating is determined by gz/g using the z axis accelerometer (gz) alone and computing the arc cosine of the averaged data. The cosine response is responsible for sensitivity problems at low inclinations. The straight averaging is responsible for the error contribution of saturation. This is because except at 90.degree. inclination, the accelerometer output is closer to saturation in one direction than the other. On average then, the accelerometer will saturate more in one direction than the other. This would have the effect of skewing the average towards zero. Equivalently, the resulting inclination error will be in the direction of 90.degree.. This is consistent with field test data.
Similarly, the rotating azimuth measurement also is error prone. The rotating azimuth calculation requires the measurement of the magnetometer z axis (hz) output while rotating. This data is combined with total magnetic field (ht) and Dip angle measurements made while not rotating, and with inclination data. The Hz measurement is analogous to the Gz measurement for inclination except that the Hz measurement can be made quite accurately. The analogy is drawn because in the absence of tool face information, the locus of possible tool orientations knowing only inclination (from gz) is a cone around vertical. The locus of tool orientations knowing hz, Dip angle and ht is also a cone. This cone is centered on the magnetic field axis. The rotating azimuth calculation is simply the determination of the direction of the horizontal projection of the intersection of these two loci. There are two lines of intersection of these two cones except at 0.degree. and 180.degree. azimuth. This produces the east-west ambiguity in the calculation. Since the angle of intersection becomes vanishingly small as the actual azimuth approaches 0.degree. or 180.degree., small errors in either cone angle measurement will result in large errors in calculated azimuth. Under some circumstances, the magnitude of this azimuth related azimuth error may be unacceptable.